The present invention relates to a process for forming seamless articles, such as tubing or zirconium or titanium material, from precursors formed by welding. The process provides a product that has uniform alpha structure throughout the article, including the weld and adjacent areas of the article.
Welded articles, such as tubing, can often be produced much more economically than seamless articles. However, seamless articles such as tubing of zirconium alloys and titanium alloys are preferred over welded tubing in critical applications, such as nuclear reactor fuel cladding (for zirconium alloys) and aircraft hydraulic lines (for titanium alloys) because of the enhanced uniformity in structure and properties of the seamless product. The non-uniformity, or heterogeneity, of the structure of a welded product is caused by the presence of the as-solidified weld structure and the weld heat-affected zones in the structure adjacent the weld.
In welded articles, the grain structure of the weld and head-affected zones could be refined by treatment in the high temperature beta phase field, but treatment of finished or near-finished-size tubing of reactive metals such as zirconium or titanium at temperatures above the beta transus, i.e. above about 950.degree. C., or 930.degree. C. respectively, is generally impractical because of oxidation and poor strength at such temperatures.
A process for improving the creep strength of Zircaloy tubes is disclosed in U.S. Pat. No. 3,865,635. According to that process, seamless tubes of a zirconium alloy are heated, before the last cold working step to the beta range and are then cooled to room temperature. The tubes were prepared in the normal way by means of melting, casting, hot and cold working up to and including the last but one cold working step. Inductive heating of the tube prior to the final cold working step was then effected, at a temperature of between 860.degree.-1250.degree. C., and the tube cold worked and finally annealed. In such preparation of tubes, seamless tubing is processed and no welds are present.
A process that teaches formation of zirconium-based alloy articles, such as hollow channels, which may be formed from welded intermediate forms, and seamless fuel cladding is described in U.S. Pat. No. 4,238,251. The articles are fabricated and then heated to the alpha-beta or beta range and quickly quenched, to produce corrosion resistant products. It is emphasized, however, in that process description, to avoid processing operatives, subsequent to the aforementioned heating and quenching, such as hot or cold rolling and annealing. An effect of such cold working or annealing is stated to be re-homogenization of the microstructural segregation produced by the process of that invention, which must be avoided in subsequent fabrication operations.
A further method for forming zirconium alloy tubes is described in U.S. Pat. No. 3,486,219. In that reference, there is disclosed a method of homogenizing the structure of a tube formed by butt welding, including reducing the wall thickness of the tube material by planatary swaging and heat treating the tube material to effect recrystallization of the structure. Treatment of Zircaloy butt welded tubes is disclosed where the tube is cold worked by planetary ball swaging and the subsequent heat treatment carried out at a temperature under the alpha-beta transformation temperature of the Zircaloy.
An object of the present invention is to produce tubing from zirconium or titanium metals or alloys by the treatment of welded precursors of such material to give a homogeneous material and subjecting the homogeneous material to forming steps to give a seamless tubing.